1. Field of the Invention
The invention relates to a display device, particularly to a display device having a pixel portion.
2. Description of the Related Art
In a liquid crystal display device, when a DC voltage is kept applied to a liquid crystal of a pixel portion for a long time, a lag phenomenon called “burn-in” occurs. Therefore, the liquid crystal display device need use a driving method of inversing a potential of a pixel electrode relative to a potential of a common electrode in a predetermine cycle. One of such driving methods of the liquid crystal display device is a DC driving method of applying a DC voltage to the common electrode. As this DC driving method, a line inversion driving method of inverting the pixel potential relative to a potential of the common electrode to be applied with a DC voltage in each of horizontal periods has been known. This technology is proposed in “Introduction to liquid crystal display engineering,” pp. 101-103, written by Yasoji Suzuki and published by the Nikkan Kogyo Shimbun, Ltd., at Nov. 20, 1998. It is noted that one horizontal period means a period of writing an image signal to all the pixel portions arrayed along one gate line.
FIG. 7 shows a waveform chart in a case where a liquid crystal display device is driven by using a conventional line inversion driving method. When the liquid crystal display device is driven by using the conventional line inversion driving method in FIG. 7, image signals are inverted relative to a potential COM of a common electrode in each of the horizontal periods. Each of the image signals is changed in each of pixel portions A to F according to an image to be displayed.
A liquid crystal display device using a dot inversion driving method of inverting each of image signals relative to a potential COM of a common electrode in each of adjacent pixel portions A to F has been proposed, too.
FIG. 8 is a waveform chart in a case where a liquid crystal display device is driven by using the conventional dot inversion driving method. When the liquid crystal display device is driven by using the conventional dot inversion driving method, different from the conventional line inversion driving method shown in FIG. 7, each of the image signals corresponding to the image to be displayed is inverted relative to the potential COM of the common electrode in each of the pixel portions A to F.
In the conventional driving methods described above, however, since a curve of transmittances of a liquid crystal layer relative to image signal voltages is highly steep as shown in FIG. 9, increasing grayscales largely narrows each of intervals between voltages to be applied to a liquid crystal for realizing the grayscales. That is, each of changing amounts of transmittances relative to the voltages (image signal voltages) is large, so that it has been difficult to realize a smooth grayscale image.